Method for purifying aqueous suspension

ABSTRACT

A method and apparatus for purifying an aqueous suspension comprising feeding an aqueous suspension containing fine particles comprising an inorganic component from the outer surface of a wavy hollow fiber membrane having an outer diameter of from 0.5 to 3.1 mm to filter, followed by physical wash of the hollow fiber membrane. The purifying method can reduce the damage of the membrane outer surface during the physical wash step, prevent open pores on the surface from covering and achieve stable filtration. The hollow fiber membrane bundle can be produced by having a pulsation flow contacted with the hollow fiber material being extruded from the double spinning nozzle under specific conditions and cooling and solidifying or coagulating it while shaking.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a method for purifying an aqueoussuspension with a module comprising wavy hollow fiber membranes.Particularly, the present invention relates to a method for purifying anaqueous suspension, for example, tap water such as river water, lakewater, marsh water and groundwater; water for industrial use: wastewater; secondary treatment waste water; industrial sewage; domesticsewage; human waste; sea water and the like with the module.

Further, the present invention is also directed to a bundle of wavyhollow fiber membranes, a process for producing the same, and a hollowfiber membrane module comprising the same.

2. Related Art

Heretofore, various methods for purifying aqueous suspensions such astap water, water for industrial use, waste water, industrial sewage,domestic sewage, human waste, sea water and the like with a hollow fibermembrane have been known. In particular, a purifying method according toso-called external pressure filtration, in which raw water permeates inthe direction from the outer to inner surface of a hollow fibermembrane, can secure a larger membrane area contributing to filtrationper unit volume when compared to so-called internal pressure filtration,in which raw water permeates in the direction from the inner to outersurface of a hollow fiber membrane. Therefore external pressurefiltration is advantageously used in the field wherein minimization ofwater production cost is required, for example, a water-treatment fieldsuch as turbidity removal for waterworks.

In the above method for purifying aqueous suspensions with hollow fibermembranes, suspended or organic materials having a size bigger than thepore diameter of the membranes, are hindered on the membrane surface tocause a so-called concentration polarization or form a cake layer.Moreover, the organic materials in aqueous suspension clog the pores ofmembranes or adhere to the inner network structure of membranes. As aresult, the filtration flux upon filtering the raw water is decreased toabout one tenth of that upon filtering pure water. Accordingly, therehave been also known purifying methods in which physical wash ofmembranes is regularly practiced after the filtration in order toachieve a more stable filtration flux.

Specifically, it has been known to carry out, after a certain-termfiltration, backward wash comprising feeding a part of filtrate in thereverse direction to the filtration, i.e., in the direction from thefiltrate side to the raw water side (hereinafter simply referred to asback wash), air-scrubbing comprising supplying compressed gases and/orcompressed air and raw water in the direction from the lower to upperpart of the hollow fiber membrane module filled with water to shakefibers and discharge suspended solids accumulating among the hollowfiber membranes from the system, and the like. For example, JapanesePatent Application Laid-Open No. 60-19002 discloses a method in which abubble generation nozzle is arranged on the side of or down the hollowfiber membranes in the hollow fiber membrane storage container, and agas is injected therefrom together with back wash.

On the other hand, as a membrane which can be used for theabove-mentioned purifying method, a reverse osmosis membrane, anultrafiltration membrane, a microfiltration membrane, a gas separationmembrane, a nanofilter, and a deairing membrane have been known. Thesemembranes are not used alone but used in plural, i.e., in the form of ahollow fiber membrane module. The membrane module is prepared bymounting a plurality of the membranes in a module case, sealing at leastone edge thereof with a thermosetting resin such as an epoxy resin, andcutting the bonded and fixed portion to open a hollow portion. Such amodule is used in various fluid treatment fields, for instance, areverse osmosis membrane module is used for desalination of sea water orbrine, production of primary pure water of ultra-pure water, andconcentration of fruit juice or milk; an ultrafiltration membrane modulefor collection of electrodeposition paints, production of pyrogen-freewater, treatment of waste water, concentration of enzymes, finalfiltration of ultra-pure water, and turbidity removal from tap water orwaste water; a microfiltration membrane module for turbidity removalfrom tap water or waste water, treatment of concentrated water, germremoval and purification of fermentation liquid, and fine particleremoval from chemicals; a gas separation membrane module for steamremoval, condensation of hydrogen, condensation or enrichment of oxygen,condensation or enrichment of nitrogen, and condensation of carbondioxide; a nanofilter module for removal of agricultural chemicals orhalogenated organic compounds; and a deairing membrane module fordeairing of water and aqueous solution. The hollow fiber membranes perse have also been studied. For example, Japanese Patent ApplicationLaid-Open No.64-22308 discloses the art using an external pressurefiltration type hollow fiber membrane module wherein hollow fibershaving wavy or spiral curls at least in a part thereof are mountedinstead of the conventional straight hollow fiber membrane in order toprevent such a mutual clinging of the hollow fibers that hinders rawwater from flowing toward the center of the module and to use almost allthe hollow fibers mounted in the module for effective filtration.

SUMMARY OF THE INVENTION

We, the present inventors, have made studies on a purifying methodenabling a stable filtration.

In particular, we have made studies focusing on the fact that the hollowfiber membrane surface is considerably damaged when the aqueoussuspension comprising fine particles containing an inorganic componentis purified according to an external pressure filtration method taking astep of physical wash such as back wash and air-scrubbing.

As a result, we have found that when an inorganic component is containedin the suspended solid accumulating among hollow fiber membranes, theouter surfaces of the membranes rub against each other through thesuspended solids at the physical wash step and pores on the surface ofthe membranes are covered with the result that the stability of thefiltration operation is deteriorated. Further, we have found that thecontinuation of such a phenomenon may result in the breakage of themembranes.

As the result of our extensive and intensive studies, we successfullyprovided, by using a bundle of hollow fiber membranes having specificwaves, diameter and further bulkiness, a purifying method enabling astable filtration, in which the damage of membrane surfaces caused by aninorganic component at the physical wash step, is unexpectedlydecreased.

Moreover, in the production of a hollow fiber membrane comprisingextruding membrane production raw liquid followed by cooling and thensolidification or coagulation, we have also succeeded in efficientlyproducing a bundle having specific waves, diameter and further bulkinessas described above by contacting a pulsation flow with hollow fibermaterials under specific conditions to vibrate and cooling andsolidifying or coagulating the hollow fiber materials while vibrating.

Namely, it is an object of the present invention to provide a purifyingmethod enabling stable filtration, in which, during the physical washstep following filtration of aqueous suspension, the external surfacesof hollow fiber membranes is prevented from rubbing against each otherthrough suspended solids in the aqueous suspension containing aninorganic component, and the covering of open pores on the surface ofhollow fiber membranes is hindered. It is also an object of the presentinvention to provide a bundle of the hollow fiber membranes which isused for the purifying method, a process for producing the bundle, and amodule mounting the bundle.

It is another object of the present invention to provide a purifyingmethod enabling a stable filtration, in which the efficiency of thephysical wash to discharge suspended solids accumulating among hollowfiber membranes is improved without damaging the surface of the hollowfiber membranes. It is also another object of the present invention toprovide a bundle which can be used for the purifying method, a methodfor producing the membrane bundle, and a module mounting the bundle.

Insufficient physical wash may cause accumulation of suspended solids inthe hollow fiber membrane bundle so that the membranes cling to eachother in the shape of a rod. In such a case, raw water cannot besupplied into the bundle with the result that the amount of the filtrateto be recovered is seriously decreased. Additionally, if physical washis conducted in the state that the hollow fiber membranes are clingingto each other in a rod shape, the hollow fiber membranes may be brokenby an excessive external force toward the horizontal direction. Thepresent invention also solves such problems.

It is another object of the present invention to provide a bundlecapable of decreasing defects caused upon bonding and fixing a bundle toa module case even if a bonding agent to be employed has a high initialviscosity before hardening or the bundle is bonded and fixed to alarge-scale module case with a large diameter, and to provide a processfor producing the bundle.

The above-mentioned objects of the present invention can be achieved bythe following.

1) A method for purifying aqueous suspension comprising feeding aqueoussuspension containing a fine particle comprising an inorganic componentfrom the outer surface of a wavy hollow fiber membrane having an outerdiameter of from 0.5 to 3.1 mm to filter, followed by physical wash ofthe hollow fiber membrane.

2) A hollow fiber membrane bundle which is prepared by collecting aplurality of wavy hollow fiber membranes so as to orient in the samedirection with a bulkiness of from 1.45 to 2.00, wherein the membranehas an inner diameter of from 0.3 to 1.7 mm, an outer diameter of from0.5 to 3.1 mm, a membrane thickness of from 0.1 to 0.7 mm, and aflatness of from 0.8 to 1.0.

3) A method for producing a hollow fiber membrane bundle comprising thesteps of:

i) extruding membrane production raw liquid in the form of a hollowfiber through a co-axial tube-in-orifice spinning nozzle to obtain ahollow fiber material,

ii) cooling and solidifying or coagulating the hollow fiber material toobtain a hollow fiber membrane, and

iii) collecting a plurality of the thus-obtained hollow fiber membranesso as to orient in the same direction;

wherein a pulsation flow is contacted with the hollow fiber materialbefore or during the cooling and solidifying step or the coagulatingstep.

4) A hollow fiber membrane module, wherein a plurality of wavy hollowfiber membranes each having an inner diameter of 0.3 to 1.7 mm, an outerdiameter of 0.5 to 3.1 mm, a membrane thickness of 0.1 to 0.7 mm and aflatness of 0.8 to 1.0 is collected so as to orient in the samedirection and mounted with a packing ratio of from 35 to 55%.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a flow chart of an example of the purifying method of thepresent invention.

FIG. 2 is a flow chart of the other example of the purifying method ofthe present invention in which an ozone gas can be added.

FIG. 3 is a schematic view of an example of a tool used for measuringbulkiness of a hollow fiber membrane bundle of the present invention.

FIG. 4 is a schematic view of an example of the method for producing ahollow fiber membrane of the present invention.

FIG. 5 is a schematic view of an example of the hollow fiber membranemodule of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

Hereinafter, the present invention is illustrated in detail.

<PURIFYING METHOD>

The purifying method of the present invention is a method for purifyingaqueous suspension comprising feeding aqueous suspension containing afine particle comprising an inorganic component from the outer surfaceof a wavy hollow fiber membrane having an outer diameter of from 0.5 to3.1 mm to filter, followed by physical wash of the hollow fibermembrane.

The filtration type can be either a dead-end type filtration wherein thewhole quantity of raw water supplied is recovered as filtrate, or across flow type filtration wherein a part of raw water supplied isrecovered as concentrated water outside the membrane module system.Also, it may be either a pressurizing filtration type wherein raw wateris pressurized from the outer surface side of the membrane by using apump or the like to obtain filtrate, or a decompressing filtration typewherein a membrane module is submerged in a raw water tank or a rawwater pit and the inner surface side of membrane is decompressed toobtain filtrate. The pressurizing filtration type is preferred because ahigher filtration flux can be obtained.

An example of filtration is shown in FIG. 1. In FIG. 1, raw water(aqueous suspension) (1) is fed under pressure into a hollow fibermembrane module (4) through a circulation tank (2) with a raw water feedpump (3). The fine particles in raw water are trapped on the outersurface of hollow fiber membranes and the resultant filtrate isintroduced into a filtrate tank (5) and stored therein.

The raw water is fed from the outer surface side of a wavy hollow fibermembrane having an outer diameter of from 0.5 to 3.1 mm. Although theouter diameter of the hollow fiber membrane can be changed depending onthe effective length of a membrane module, the expected quantity offiltrate, or the like, is required to be within the range of from 0.5 to3.1 mm in view of the pressure loss of filtrate in the hollow part ofthe membrane or the like. The outer diameter is preferably within therange of from 0.7 to 2.5 mm, more preferably within the range of from1.0 to 2.5 mm.

In the present invention, wavy hollow fiber membranes are collected inthe longitudinal direction so as to be bulky and mounted in a module.Therefore, the hollow fiber membranes contact almost at points andhardly rub against each other through suspended solids so that openpores on the membrane surface are not easily covered. As a result, astable filtration operation is achieved. For the same reasons, suspendedsolids hardly accumulate in a hollow fiber membrane bundle, and even ifthe solids accumulate, they are easy to discharge by taking a step ofphysical wash such as back wash, air-scrubbing and flushing so that astable filtration operation over a long period can be achieved.

The raw water is not particularly limited as far as it Is an aqueoussuspension containing fine particles comprising an inorganic component.It includes river water, lake water, marsh water, ground water,reservoir water, secondary treatment waste water, industrial sewage,waste water and the like. The fine particles comprising an inorganiccomponent mean a suspended solid element in aqueous suspension, andindicate metals such as iron, manganese, aluminum and silicon; oxidesthereof; oxide compounds thereof; and/or their condensation particlewith organic compounds such as humic acid and fulvic acid. The fineparticle includes a particle having a diameter of from 0.1 to 500 μm.When raw water to be fed into a module contains a big particle having adiameter more than 500 μm like river water, pre-treatment like screenmesh is generally conducted to prevent the big particle from being fedinto a module. The particle diameter is a value measured by using aparticle size distribution measuring apparatus of laserdiffraction/scattered type, LA-910 (trade name) manufactured and sold byHoriba, Ltd.

The purifying method of the present invention is effective especially inthe situation where a suspended solid accumulating on the surface ofmembranes during the filtration has a large diameter and theaccumulation quantity of the solids is large. This is because, in such asituation, the membrane surface is most seriously damaged by the solidsupon physical wash.

The amount of water to be filtered and the filtration time areappropriately adjusted according to the turbidity of raw water (aqueoussuspension). As the turbidity of raw water becomes higher, it isnecessary to reduce the amount of water to be filtered or shorten thetime until the physical wash. Further, as the amount of raw waterbecomes larger, it is necessary to shorten the time until the physicalwash. In Particular, for the purpose of preventing the suspended solidsaccumulating among the hollow fiber membranes from hardening andadhering to each other, it is preferred to select a filtration time soas for the amount of the suspended solids accumulating, which is definedby the following formula, to be in the range of from 0.0005 to 10,moreover in the range of from 0.01 to 10. The amount of suspended solidsaccumulating is a parameter for the amount of suspended solidsaccumulating on the unit membrane surface during a filtration step andis defined by the following formula:

Amount of suspended solids accumulating=(Raw water turbidity[degree])×(Total amount of filtrate permeating membrane in filtrationtime [m³])/(Membrane surface area [m²])

The turbidity of raw water in the above formula means an averageturbidity among days, and can be obtained by measuring the turbidity forplural days according to JIS K0101 9.2 and averaging the obtainedvalues.

In the purifying method of the present invention, the filtration asdescribed above is followed by physical wash such as back wash,air-scrubbing and flushing.

The back wash is an operation comprising feeding a part of filtrateand/or a compressed gas from the filtrate side of a hollow fibermembrane (the inner surface side in case of the external pressure typefiltration) to the raw water side (the outer surface side In case of theexternal pressure type filtration) to generate a flow of liquid and/orgas in the reverse direction to the ordinary filtration flow. Forexample, in FIG. 1, washing (back wash) is performed by feeding thefiltrate in a filtrate tank (5) into a hollow fiber membrane module (4)with a back wash pump (6).

Each time necessary for a filtration step and a back wash step isappropriately selected according to the quality of raw water, theexpected amount of filtrate, or the like. It is preferred that the timeof the back wash step is from {fraction (1/10000)} to ⅕ of that of thefiltration step. When the time of the back wash step is shorter than{fraction (1/10000)} of that of the filtration step, the effect of theback wash can be deteriorated. When the time of the back wash step islonger than ⅕ of that of the filtration step, a filtration time per unittime becomes short. As a result, the recovery ratio of filtrate may bedecreased when filtrate is used for back wash.

In view of the balance of a recovery ratio of filtrate and membranerecoverability by physical wash, water and/or compressed gas for backwash flows preferably in a flow amount [m³/Hr] of from 0.5 to 5 times,particularly preferably in a flow amount [m³/Hr] of from 1 to 3 times,as large as the flow amount [m³/Hr] of filtrate during the filtrationstep.

The air-scrubbing step is an operation comprising feeding raw watercontaining compressed gas such as compressed air and/or only compressedgas from the downside of a hollow fiber membrane module between thefiltration steps to discharge the suspended solids accumulating amongthe hollow fiber membranes from the module. For example, in FIG. 1,air-scrubbing is performed by feeding compressed air generated in acompressor (7) into a raw water inlet of a hollow fiber membrane module(4). When the air-scrubbing step is carried out alone between thefiltration steps using a conventional hollow fiber membrane module, themembrane surface may be damaged and the open pores on the surface may becovered if the amount of suspended solid accumulating per unit membranearea is large at the time of conducting air-scrubbing. According to thepresent invention, however, the treated water having high quality can bestably obtained at a high flow velocity of the membrane filtration evenif severe air-scrubbing as described above is performed alone.

Respective times necessary for a filtration step and an air-scrubbingstep are appropriately selected according to the quality of raw water,the expected amount of filtrate, or the like. It is preferred that thetime of the air-scrubbing step is from {fraction (1/10000)} to ⅕ of thatof the filtration step. When the time of the air-scrubbing step isshorter than {fraction (1/10000)} of that of the filtration step, theeffect of air-scrubbing can deteriorate. When the time of theair-scrubbing is longer than ⅕ of that of the filtration step, theproportion of the air-scrubbing step time to the total operation timebecomes large. As a result, the amount of filtrate recovered per unittime is decreased.

The flow amount [Nm³/Hr] of gas fed in the normal state during theair-scrubbing step is preferably from 0.5 to 20 times, more preferablyfrom 1 to 10 times, as large as the flow amount [m³/Hr] of filtrateduring the filtration step. The effect of air-scrubbing may bedeteriorated when the flow amount is under the lower limit, and thehollow fiber membranes may be dried when the flow amount is over theupper limit.

The flushing step is an operation comprising widely opening a valve onthe condensed water side and/or an air exhausting valve and feeding rawwater in an amount larger than that in the filtration step to dischargethe suspended solids accumulating among the hollow fiber membranes fromthe module. In this step, the valve on the filtrate side may be closedor throttled. Respective times necessary for a filtration step and aflushing step are appropriately selected according to the quality of rawwater, the expected amount of filtrate or the like. It is preferred thatthe time of the flushing step is from {fraction (1/10000)} to ⅕ of thatof the filtration step. When the time of the flushing step is shorterthan {fraction (1/10000)} of that of the filtration step, the effect offlushing may deteriorate. When the time of the flushing step is longerthan ⅕ of that of the filtration step, the proportion of the flushingstep time to the total operation time becomes large. As a result, theamount of filtrate recovered per unit time is decreased.

In view of the balance of a recovery ratio of filtrate and membranerecoverability by physical wash, a flushing amount [m³/Hr) of waterduring the flushing step is preferably from 1.1 to 8.0 times, morepreferably from 1.5 to 5.0 times, as large as the flow amount [m³/Hr] offiltrate during the filtration step.

The above-mentioned physical wash may be performed alone or incombination. Air-scrubbing simultaneous with back wash enables a stablerand longer-term filtration operation because it releases the compactionof suspended solids accumulating on the membrane surface and makes thesolids float to discharge by air-scrubbing. It is also acceptable toperform back wash alone prior to air-scrubbing or air-scrubbingsimultaneous with back wash. In this case, the release of the compactionof suspended solids accumulating on the membrane surface isadvantageously accelerated. It is also acceptable to perform back washalone after air-scrubbing or air-scrubbing simultaneous with back wash.In this case, the discharge of suspended solids in a membrane module isadvantageously accelerated. Further, the physical wash method, in whichflushing is performed after back wash and air-scrubbing aresimultaneously performed, can be one of the effective physical washmethods because the recovery ratio of filtrate is improved by subjectinga part of suspended solids discharged by back wash and air-scrubbing toflushing.

The purifying method of the present invention can employ a step ofdosing ozone or the like in addition to the above-mentioned filtrationstep and physical wash step. One example of such a case is shown in FIG.2. As shown in FIG. 2, raw water (11) is introduced into a circulationtank (12), fed into a hollow fiber membrane module (14) under pressureby using a raw water supplying pump (13), filtered in the module, andthen stored in a filtrate tank (15). At this time, the raw water beingfed into the module (14) under pressure is mixed with ozone gasgenerated by an ozone generator (18). The concentration of ozone wateris controlled to be a certain concentration, for example 0.3 mg/l, onthe filtrate side. At the time of back wash, the filtrate in thefiltrate tank (15) is transferred to the module (14) by a back wash pump(16). At this time, air-scrubbing with compressed air generated by acompressor (17) may be performed.

<HOLLOW FIBER MEMBRANE BUNDLE>

The hollow fiber membrane bundle used in the above-mentionedpurification method is preferably a bundle which is prepared bycollecting a plurality of wavy hollow fiber membranes so as to orient inthe same direction with a bulkiness of from 1.45 to 2.00, wherein themembrane has an inner diameter of from 0.3 to 1.7 mm, an outer diameterof from 0.5 to 3.1 mm, a thickness of from 0.1 to 0.7 mm, and a flatnessof from 0.8 to 1.0.

The material for a hollow fiber membrane includes polyolefin such aspolyethylene, polypropylene, polybutene and the like; fluoro type resinsuch as a tetrafluoroethylene-perfluoroalkyl vinyl ether copolymer(PFA), a tetrafluoroethylene-hexafluoropropylene copolymer (FEP), atetrafluoroethylene-hexafluoropropylene-perfluoroalkylvinylethercopolymer (EPE), an ethylene-tetrafluoroethylene copolymer (ETFE),polychlorotrifluoroethylene (PCTFE), chlorotrifluoroethylene-ethylenecopolymer (ECTFE), polyvinylidene fluoride (PVDF) and the like;super-engineering plastics such as polysulfone, polyether sulfone,polyether ketone, polyether ether ketone, polyphenylene sulfide and thelike; cellulose such as cellulose acetate, ethyl cellulose and the like;polyacrylonitrile, polyvinyl alcohol and compositions thereof.

As the hollow fiber membrane, a membrane having a pore diameter in therange of a nanofilter, an ultrafiltration (UF) membrane and amicrofiltration (MF) membrane can be used. Among these, theultrafiltration (UF) membrane and the microfiltration (MF) membranewhich basically contribute a high filtrate amount are preferred. Inparticular, the MF membrane is more preferred. For example, thepreferred membrane has an average pore diameter of from 0.001 to 1 μm,and the more preferred membrane has that of from 0.05 to 1 μm. Herein,the average pore diameter is measured by an air flow method (ASTM:F316-86). Further, a hollow fiber membrane having a porosity of from 50to 90% is preferred. Herein, the porosity is calculated from a weight ofa hollow fiber membrane saturated with water, a simple volume of ahollow fiber membrane (a volume calculated from the inner diameter,outer diameter and length thereof) and a specific gravity of the polymerused.

The hollow fiber membrane of the present invention is wavy. The wavyhollow fiber membrane means a membrane which meanders when being leftwithout applying any tension.

From the viewpoint of improving the bulkiness of a hollow fiber membranebundle, it is preferred that the waves having different wavelengthand/or wave height exist together.

The hollow fiber membrane bundle has a bulkiness of preferably from 1.45to 2.00, more preferably from 1.55 to 2.00.

The bulkiness of the hollow fiber membrane mentioned above is defined bythe following formula.

Bulkiness=(S1/S2)  (I)

In the above formula (I), S1 represents cross-section area of a smallbundle of six hundred hollow fiber membranes, which are selectedrandomly from hollow fiber membranes constituting a membrane bundle,under load of 2.9 N (300 gf). S1 can be calculated from thecircumferential length of the small bundle measured under load of 2.9 N(300 gf) using a polyethyleneterephthalate (PET) film, which has athickness of 100 μm and width of 40 mm, and is equipped with a springscale at the edge. S2 represents a value which is obtained bymultiplying the cross-sectional area of a hollow fiber membranecalculated using the outer diameter thereof by six hundred.

Further, the circumferential length of a hollow fiber membrane bundlecould be more easily measured by using a tool for the bulkinessmeasurement prepared by joining two PET films through a rectangularframe in advance. A specific example of the measuring methods isexplained referring to FIG. 3.

i) One of the two PET films (21), which are jointed through arectangular frame (22), is passed through the frame (22) to make acylinder.

ii) Into the cylinder, a hollow fiber membrane bundle (24) is installed.After fixing one of the two PET films, 2.9 N (300 gf) weight was appliedto the other PET film with a hole (23), which is arranged in advance forholding the hook of a spring scale, through a spring scale to mark thePET film.

iii) The PET film is taken off and the circumferential length ismeasured from the marking. At this time, it is preferred that the PETfilm is graduated because the circumferential length is directlymeasured.

When the bulkiness is less than 1.45, the uniformity of a fillingadhesive agent at the time of preparing a module and the efficiency ofdischarge of suspended solids by physical wash after a module is madecan be insufficient. When the bulkiness is more than 2.00, a maximumnumber of hollow fiber membranes to be installed in a module candecrease so that it can be difficult to obtain a sufficient membranearea per module.

The outer diameter of the hollow fiber membrane is within the range offrom 0.5 to 3.1 mm, preferably within the range of from 0.7 to 2.5 mm,more preferably within the range of from 1.0 to 2.5 mm, from theviewpoint of the amount of filtrate to be recovered, the efficiency ofcompressive creep, the necessary bursting pressure or the like. Inaddition, the inner diameter of the hollow fiber membrane is preferablywithin the range of from 0.3 to 1.7 mm. In case of the external pressurefiltration, since filtrate flows in the hollow portion of the hollowfiber membranes, the inner diameter of less than 0.3 mm causes theincrease of pressure loss of filtrate in the hollow portion so that afiltrate amount to be recovered can be reduced with the decrease of theeffective membrane area in a module. On the other hand, when the innerdiameter is more than 1.7 mm, it is difficult to obtain a large membranearea per module so that a filtrate amount to be recovered can bedecreased as well. The thickness of the hollow fiber membrane ispreferably within the range of from 0.1 to 0.7 mm, more preferablywithin the range of from 0.2 to 0.5 mm, from the viewpoint of thebalance of the compressive creep efficiency and necessary burstingpressure, which are exhibited when the inner diameter of the membrane iswithin the above-mentioned range.

The flatness of the hollow fiber membrane is preferably from 0.8 to 1.0.Herein, the flatness means a ratio of the inner minor axis to the innermajor axis, especially the ratio at the curved portion of a wavymembrane, when the cross-section of the hollow fiber membrane is oval.The ratio is defined by the following formula.

Flatness=Minor Axis/Major Axis

The flatness may be calculated either from a value measured only at onepoint, or from an average of respective values of the minor axis andmajor axis measured at certain points, for example 5 points.

When the flatness is less than 0.8, the burst strength or thecompressive strength can be greatly decreased, and the pressure loss ofthe expansion and the reduction can be repeated when the filtrate passesthrough the waved portions of the hollow fiber membrane. As a result,the operation pressure at the time of filtration operation can beincreased and the stability of filtration can be defective. The flatnessis preferably not less than 0.9, more preferably not less than 0.95, toimprove the bursting strength and the compressive strength and suppressthe rise of pressure loss.

It is preferred that the hollow fiber membrane bundle of the presentinvention consists of wavy hollow fiber membranes in order to reduce thecontact portion where the hollow fiber membranes in contact each otherand to prevent the covering of open pores on the membrane surface causedby scrubbing of the membrane surfaces.

<METHOD FOR PRODUCING HOLLOW FIBER MEMBRANE BUNDLE>

The hollow fiber membrane bundle of the present invention can bepreferably obtained by a method for producing a hollow fiber membranebundle comprising the steps of extruding membrane production raw liquidin the form of a hollow fiber through a co-axial tube-in-orificespinning nozzle to obtain a hollow fiber material, cooling andsolidifying or coagulating the hollow fiber material to obtain a hollowfiber membrane, and collecting a plurality of the thus-obtained hollowfiber membranes so as to orient in the same direction; wherein apulsation flow is contacted with the hollow fiber material before orduring the cooling and solidifying step or the coagulating step.

One of the examples of this method is shown in FIG. 4. Hereinafter, themethod of the present invention is explained referring to FIG. 4.

One of the features of the present invention is to contribute waves to amembrane by contacting a hollow fiber material (32) extruded from thespinning nozzle (31) to flow down in a half-solidified state with apulsation flow injected from a pulsation flow exhalation nozzle (33).Namely, by making a pulsation flow contacted with a half-solidifiedhollow fiber material, the hollow fiber material is vibrated. In acurved state, the material is cooled and solidified or coagulated in acooling and solidifying bath or a coagulation bath. As a result, wavescan be formed easily.

The wavy hollow fiber membrane can be also obtained by other methodsthan above, for example, heat treatment of the hollow fiber membrane orthe like. In case of a hollow fiber membrane having a large diameter,however, the hollow portion of the membrane can be unpreferably crushedor flattened in excess when it is tried to form waves according to theabove-mentioned heat treatment. On the contrary, the production methodof the present invention employing the above-mentioned pulsation flowcan contribute waves without causing any flatness of inner/outerdiameters even if the hollow fiber membrane has a large diameter, forexample, an outer diameter of even 2.5 mm. In addition, preferred waveshaving different wavelength and wave height can be easily formed.Further, it is possible to prevent a damage of the membrane surfacebecause solids do not contact with the membrane surface. According tothe production method of the present invention employing a pulsationflow, the flatness of the hollow fiber membrane obtained can be from 0.8to 1.0, in most cases be from 0.9 to 1.0.

The pulsation flow in the present invention means to supply a fluid suchas a gas or a liquid at constant intervals. When a gas is used as afluid, a pulsation flow can be supplied by opening and shutting anelectromagnetic valve or the like of a pressure vessel under pressure.When a liquid is used as a fluid, a pulsation flow can be supplied byexhaling the fluid at constant intervals by using a bellows pump, adiaphragm pump, a plunger pump, a gear pump or the like. In this case,as shown in FIG. 4, a process in which the liquid per se for a coolingand solidification bath or a coagulation bath (34) is circulated with adiaphragm pump(35) and used for a pulsation flow is most preferablesince it is a simple process not causing any impurities.

The recurrence interval of a pulsation flow (exhalation interval) isproperly adjusted according to a winding speed of a hollow fibermembrane in the membrane production. For instance, when the windingspeed is within. the range of from 10 to 30 m/min, the recurrenceinterval is preferably within the range of from 0.05 to 1.5 sec/shot.When the interval is shorter than 0.05 sec/shot, the pulsation flow doesnot occur with the result that waves are not formed. When the intervalis longer than 1.5 sec/shot, the resultant waves have a long wavelengthwith the result that the bulkiness is insufficient.

The temperature of a pulsation flow is not especially limited. Thistemperature can be the temperature of a cooling and solidifying bath,for example, within the range of from 20 to 80° C. in the case of themembrane production process by melt extrusion, and a temperature of acoagulating bath, for example, within the range of from −10 to 80° C. inthe case of the wet membrane production.

The waves can be formed by contacting a half-solidified hollow fibermaterial with a pulsation flow to shake, and cooling and solidifying orcoagulating the material in the state of shaking. It is supposed thatone contact of a pulsation flow with the hollow fiber material forms notmerely one wave, but 2 to 10 waves. The half-solidified hollow fibermaterial meanders by the contact with a pulsation flow and themeandering attenuates gradually. Therefore, the resultant hollow fibermembrane does not have one kind of the wavelengths and/or wave heights,but various kinds of wavelengths and/or wave heights together.

When the pulsation flow contacts the half-solidified material during thecooling and solidifying or coagulating step, that is, in a cooling andsolidifying bath or a coagulating bath, the contact position ispreferably, for example, from the bath surface to not deeper than 500 mmbelow the bath surface. When the pulsation flow contacts thehalf-solidified material before the cooling and solidifying step or thecoagulating step, that is, above the bath surface of the cooling andsolidifying bath or the coagulating bath, the contact position ispreferably, for example, from the bath surface to not higher than 50 mmabove the bath surface. When the contact position is far above thecooling and solidifying bath or the coagulating bath, i.e., near thespinning nozzle, only the portion where the pulsation flow contacts ispromptly cooled and solidified or coagulated so that it is possible thatpores may not be uniformly formed on the circumference of the hollowfiber membrane and, in an extreme case, no pores may be formed at thecontact portion or the skin layer may become thick only at the contactportion. On the contrary, when the contact position is deep in the bath,e.g., deeper than 500 mm below the bath surface, the hollow fibermaterial is cooled and solidified or coagulated before the contact withthe pulsation flow, and waves are not formed.

If a guide such as a thread guide is arranged in the cooling andsolidifying bath or the coagulating bath so as for the hollow fibermaterial not to run off by the contact with the pulsation flow at thetime when the pulsation flow is contacted with the hollow fibermaterial, more preferred waves can be formed.

<HOLLOW FIBER MEMBRANE MODULE>

In the purifying method of the present invention, it is preferred to usea hollow fiber membrane module in which a plurality of wavy hollow fibermembranes each having an inner diameter of 0.3 to 1.7 mm, an outerdiameter of 0.5 to 3.1 mm, a thickness of 0.1 to 0.7 mm and a flatnessof 0.8 to 1.0 is collected so as to orient in the longitudinal directionand mounted with a packing ratio of from 35 to 55%.

The packing ratio means a ratio at which the inner wall sectional areaof a module case is packed with hollow fiber membranes based on theouter diameter of the membrane, which can be calculated by the followingformula.

Packing ratio (%)=(Sectional area based on outer diameter of hollowfiber membrane×Packing number of hollow fiber membrane permodule)×100/(Sectional area based on inner wall of module case)

By installing a hollow fiber membrane bundle having high bulkiness at apacking ratio of from 35 to 55%, the contact of the hollow fibermembranes remains only at points since the bundle therein is bulky. As aresult, the hollow fiber membranes hardly rub against each other throughsuspended solids and open pores on the outer surface of the hollow fibermembrane are not easily covered. For the same reasons, the suspendedsolids hardly accumulate in the hollow fiber membrane bundle and areeasily discharged by physical wash such as back wash, air-scrubbing,flushing or the like, even if accumulate. As a result, a filtrationoperation can be stably conducted for a long period. Further, since thedistribution situation of the hollow fiber membranes in the innersection of the membrane module improves owing to the waves, thedefective portion hardly occurs in the bonded and fixed portion of themodule even in the case that the pre-hardening initial viscosity of thebonding agent is high or the module is a large-scale module having alarge diameter when the hollow fiber bundle is bonded and fixed to amodule case.

Although the packing ratio of less than 35% provides excellentefficiency of discharge by wash, the effect of using a hollow fibermembrane module is reduced since a large membrane area per unit volumeof the hollow fiber module is not secured. The packing ratio of morethan 55% can secure a large membrane area per unit volume of the hollowfiber module, but the hollow fiber membranes aggregate densely in themodule so that the suspended solids are hard to discharge in case of theexternal pressure filtration.

In the hollow fiber membrane module of the present invention, at leastone edge of the hollow fiber membrane bundle is fixed with athermosetting resin like an epoxy resin. The hollow fiber membranemodule of the present invention is mounted with hollow fiber membraneswith a hollow portion open and has a structure enabling a filtrationfrom the outer to inner surface of the hollow fiber membrane; therefore,it is suitable for an external pressure filtration. The hollow fibermembrane module may be bonded and fixed at both edges or either edge. Itis also allowed to seal the hollow portion of hollow fiber membranes atone of the bonded and fixed edges. It is also possible to use a membranemodule in which both edges are bonded and fixed, the hollow portions ofthe hollow fiber membranes are sealed at one edge and an inlet for rawwater is opened as described in Japanese Patent Application Laid-OpenNo. 7-171354.

The hollow fiber membrane module of the present invention includes acartridge type module which is used after being installed and arrangedin a tank with tube sheets or an outline housing beside a directlyconnected rack type, which is connected to a rack through pipes or thelike. The above-mentioned cartridge type module indicates, differingfrom the common directly connected rack type module, such a module thatmaintains the shape of a hollow fiber membrane bundle portion with acylinder provided with holes by punching or the like, a net and thelike, and does not take fluid-tight treatments except for the bonded andfixed portion. In this case, the hollow fiber membrane bundle is allowedto be naked except for the bonded and fixed portion if the bundle canmaintain its shape by itself.

Since the hollow fiber membrane module of the present invention employsthe wavy hollow fiber membranes as described above, the bonded and fixedportion at the edge of the hollow fiber membranes is less defective evenin the case of a large-scale module having an outer diameter of from 170to 350 mm.

The thermosetting resin used to bond and fix one or both edges of thehollow fiber membrane module of the present invention includes an epoxyresin, a urethane resin, a silicone rubber and the like. If necessary,there may make attempts to improve the strength of a resin partition andreduce shrinkage on curing by adding a filler such as carbon black andfluorocarbon to these resins.

The material of the hollow fiber membrane module case includespolyolefins such as polyethylene, polypropylene and polybutene; fluororesins such as polytetrafluoroethylene (PTFE), PFA, FEP, EPE, ETFE,PCTFE, ECTFE, PVDF and the like; chloro resins such as polyvinylchloride and polyvinylidene chloride; a polysulfone resin, apolyethersulfone resin, a polyallyl sulfone resin, a polyphenyl etherresin, an acrylonitrile butadiene styrene copolymer resin (ABS resin),an acrylonitrile styrene copolymer resin, a polyphenylene sulfide resin,a polyamide resin, a polycarbonate resin, a polyether ketone resin, apolyether ether ketone resin, compounds thereof, and metals such asaluminum and stainless steels. In addition, compounds of resin andmetals, resin reinforced with glass fiber or carbon fiber can be used.

The hollow fiber membrane module of the present invention can beprepared, for example, by bonding and fixing at least one edge of thehollow fiber bundle collected in the longitudinal direction with athermosetting resin such as an epoxy resin and then cutting a part ofthe bonded and fixed portion so as to open the hollow portion of thehollow fiber membrane.

A plurality of openings is preferably arranged at one of the edge bondedand fixed portions of the hollow fiber membrane module to supply rawwater and/or gas for air-scrubbing more uniformly. The above-mentionedopening preferably has an equivalent diameter of 3 to 100 mm. When thediameter of the opening is less than 3 mm, the opening can be cloggedwith suspended solids included in the raw water. When the diameter ofthe opening is more than 100 mm, it is required to reduce the number ofhollow fiber membranes installed in the module and/or the number of theopenings so that the raw water is hard to be supplied uniformly. Thesection configuration of the openings is not especially limited, andincludes polygons such as a triangle, quadrangle, hexagon and the likein addition to circle and oval. Among them, circle and oval arepreferred. Further, the openings can be arranged uniformly or at randomat the edge bonded and fixed portions.

One of the examples of the hollow fiber membrane module of the presentinvention is shown in FIG. 5. In FIG. 5, a hollow fiber membrane moduleis connected with a pipe of an operation device through a cap (46). Theraw water and/or compressed gas to be supplied pass through a raw waterinlet (45) and are filtered from the outer to inner surface of the wavyhollow fiber membrane (41).

In this case, the pressure of the raw water pressured by a pump or thelike is maintained by a module case (43), and a part of the raw water isrecovered as filtrate. The condensed raw water is discharged from thehollow fiber membrane module through a condensed water outlet (47). Thehollow fiber membranes with hollow portion open are bonded and fixedfluid-tight to the module case at the bonded portion (42) so as not tomix raw water and filtrate. At a bonded portion (44), the hollowportions of the hollow fiber membranes are sealed and at the same timeare equipped with a plurality of openings, and a raw water inlet (45) isarranged.

According to the purifying method of the present invention, it ispossible to prevent damage of the membrane surface by fine particles atthe time of treating the aqueous suspension containing fine particlescomprising an inorganic material with the membrane and to stably performa filtration over a long term. Accordingly, the present invention issuitable for the field of purifying the aqueous suspension containing aninorganic material, e.g., tap water such as river water, lake water,marsh water and ground water; water for industrial uses; waste water;secondary treatment waste water; industrial sewage; domestic sewage;human waste; sea water and the like. In addition, the hollow fibermembrane module of the present invention has an advantage of lessscrubbing and damage to the membranes due to bulky waved hollow fibermembranes having a large diameter. Therefore, it can be suitably usedfor the purifying method of the present invention. The module also hasexcellent discharge efficiency of suspended solids. Further, the hollowfiber membrane of the present invention is suitable for a large-scalemodule which has fewer defects in the bonded portion at the edge ofmembrane.

Hereinafter, examples of production of the hollow fiber membrane, thehollow fiber membrane module and the aqueous suspension purifyingmethod, which are employed in the present invention, are described. Inthe Examples and Comparative Examples, turbidity and particle size weremeasured by the following method.

Flatness of Hollow Fiber Membrane: A curved portion of the wave of thehollow fiber membrane was cut out at five points to measure the minoraxis and major axis of the inner diameter thereof using an X-Ymicroscope [STM-222DH (trade name) manufactured and sold by OlympusOptical Company Limited], and flatness (minor axis/major axis) of eachportion was calculated.

Turbidity: Measured according to JIS K 0101 9.2 using a measuringapparatus manufactured and sold by Shimadzu Corporation 50 mm cell,UV-160A].

Particle Size: Measured using a particle size distribution meter [LA-910(trade name) manufactured and sold by Horiba, Ltd.].

Water Flux Amount of Single Hollow Fiber Membrane: Pure water at 25° C.is permeated from the inner to outer surface side of a porous hollowfiber membrane sample having an effective length of 100 mm to calculatea flux amount per unit time and that per unit pressure (differentialpressure per unit membrane).

Wash Recoverability: Evaluated based on a ratio (%) of a pure water fluxamount of a module after subjected to evaluation by real liquid such asriver water and chemical wash to a pure water flux amount (initialvalue) of a module before subjected to evaluation by real liquid; or aratio (%) of a pure water flux amount of a single membrane fiber, whichis obtained by dismantling a module after evaluation by real liquid andwashing only the membrane with chemicals, to a pure water flux amount (aflux amount of an unused membrane) of a single hollow membrane fiberbefore preparing a module.

EXAMPLE 1 (PRODUCTION OF HOLLOW FIBER MEMBRANE)

40.0 parts by weight of a powdery PVDF [KF#1000 (trade name)manufactured and sold by Kureha Chemistry Co., Ltd.], 23.0 parts byweight of hydrophobic silica [Aerosil R-972 (trade name) manufacturedand sold by Nippon Aerosil Co., Ltd.; average primary particle diameter:0.016 μm, specific surface area: 110 m²/g, Mw value (methanolwetability, volume %): 50%], 30.8 parts by weight ofdi-(ethyl-hexyl)-phthalate (DOP) [CS sizer (trade name) manufactured andsold by Chisso Corporation], and 6.2 parts by weight ofdi-butylphthalate (DBP) [manufactured and sold by Chisso Corporation]were mixed by a Henschel mixer. The resultant blend was extruded andpelletized by means of a twin-screw extruder.

The thus-obtained pellets were melt extruded into a cooling andsolidifying bath at 40° C. (hot water at 40° C.), which was placed 30 cmbelow the spinning nozzle, from a twin-screw extruder having a barreltemperature of 260° C., a head temperature of 235° C. and a spinningnozzle temperature of 230° C. through a co-axial tube-in-orificespinning nozzle having a size of inner diameter of outside nozzle/outerdiameter of inside nozzle/inner diameter of inside nozzle =1.70 mmφ/0.90mmφ/0.50 mmφ.

At the time of the extrusion, a pulsation flow exhalation nozzle wasarranged at the position of 10 mm above the bath surface, and a coolingand solidifying liquid was contacted with the hollow fiber materialflowing down at an exhalation interval of 0.3 sec/shot using a diaphragmpump [NDP-5FST manufactured and sold by Yamada Corporation] to obtain awavy hollow fiber membrane.

The above-mentioned wavy hollow fiber membrane was wound up through athree-ream roller at a winding speed of 20 m/min. The obtained hollowfiber membrane bundle was treated with dichloromethane under thefollowing conditions to extract DOP and DBP from the hollow fibermembrane.

Extraction Conditions:

Treatment Temperature: room temperature (25 to 27° C.)

Volume of dichloromethane relative to simple volume of hollow fibermembrane (calculated from inner diameter, outer diameter and lengththereof): 20 fold

Treatment Period: 5 hours

Then, the obtained hollow fiber membrane bundle was soaked in a 50%ethanol solution for 30 minutes and treated with a sodium hydroxidesolution having a weight percent concentration of 20% under thefollowing conditions to extract silica from the hollow fiber membrane.

Extraction Conditions:

Temperature: 60° C.

Volume of sodium hydroxide solution relative to simple volume of hollowfiber membrane (calculated from inner diameter, outer diameter andlength thereof): 20 fold (8 fold equivalent in equivalent ratio relativeto hydrophobic silica)

Treatment Period; 2 hours

The above treated hollow fiber membrane bundle was rinsed for an hourwith 60° C. hot water having the same volume as the above-mentionedsodium hydroxide solution. The above wash with hot water was repeated atotal of ten times to obtain a porous hollow fiber membrane bundle. Thethus-obtained hollow fiber membrane had an inner diameter/outer diameterof 0.70 mmφ/1.25 mmφ, a porosity of 70%, an average pore diameter of0.18 μm, a pure water flux amount of 2,000 [l/m²·min·100 kPa·25° C.],and flatness as shown in Table 1. The circumferential length and thebulkiness of the hollow fiber membrane bundle were 124.0 mm and 1.66,respectively. In addition, waves with different wavelength and waveheight coexisted in the hollow fiber membrane.

EXAMPLE 2 (PRODUCTION OF HOLLOW FIBER MEMBRANE)

A hollow fiber membrane bundle was prepared in substantially the samemanner as described in Example 1 except that the pulsation flow was notcontacted with the hollow fiber. The thus-obtained hollow fiber membranehad an inner diameter/outer diameter of 0.70 mmφ/1.25 mmmφ, a porosityof 70%, an average pore diameter of 0.18 μm, a pure water flux amount of2,000 [l/m² ·min·100 kPa·25° C.] and flatness as shown in Table 1. Inaddition, the circumferential length and the bulkiness of the hollowfiber membrane bundle were 115.0 mm and 1.43, respectively.

EXAMPLE 3 (PRODUCTION OF HOLLOW FIBER MEMBRANE)

The hollow fiber membrane obtained in Example 2 was passed between twogears at the atmosphere temperature of 140° C. to obtain a wavy hollowfiber membrane bundle. The gears used had a curvature minimum radius atthe edge of 5 mm and an edge distance of 25 mm. Two of such gears werebitten each other so as to be a biting height and minimum gear distanceof 15 mm and 3 mm, respectively. The thus-obtained hollow fiber membranehad an inner diameter/outer diameter of 0.70 mmφ/1.25 mmφ, a porosity of70% and an average pore diameter of 0.18 μm. Its pure water flux amountwas slightly reduced to 1,950 [l/m² ·min·100 kPa·25° C.], which wassupposed to be caused by an influence of the flatness of the hollowfiber membrane as shown in Table 1. Further, the circumferential lengthand bulkiness of the hollow fiber membrane bundle were 118.2 mm and1.51, respectively. In addition, waves of the hollow fiber membrane hadapproximately the same wavelength and wave height.

EXAMPLE 4 (PRODUCTION OF HOLLOW FIBER MEMBRANE)

The hollow fiber membrane obtained in Example 2 was passed between twogears at the atmosphere temperature of 140° C. to obtain a wavy hollowfiber membrane bundle. The gears used had a curvature minimum radius atthe edge of 7 mm and an edge distance of 30 mm. Two of such gears werebitten each other so as to be a biting height and minimum gear distanceof 10 mm and 8 mm, respectively. The thus-obtained hollow fiber membranehad an inner diameter/outer diameter of 0.70 mmφ/1.25 mmφ, a porosity of70% and an average pore diameter of 0.18 μm. Its pure water flux amountwas 2,000 [l/m² ·min·100 kPa·25° C.], and the flatness of the hollowfiber membrane as shown in Table 1. Further, the circumferential lengthand bulkiness of the hollow fiber membrane bundle were 115.5 mm and1.44, respectively. In addition, waves of the hollow fiber membrane hadapproximately the same wavelength and wave height.

EXAMPLE 5 (PRODUCTION OF HOLLOW FIBER MEMBRANE MODULE) 300 of the hollowfiber membranes obtained in Example 1 were bundled.

Then, the hollow portion of one of the edge faces of the thus-obtainedbundle was sealed and the bundle was mounted in a cylindricalpolysulfone module case having an inner diameter of 36 mmφ and a lengthof 1,000 mm. On the sealed edge of the hollow fiber membrane, only abonding jig was attached fluid-tight to the module case. On the otheredge of the hollow fiber membrane, a total of five polypropylene rods,each having an outer diameter of 5 mmφ, were arranged in parallel to thehollow fiber membrane and then a bonding jig was attached fluid-tight tothe module case. In this case, the packing ratio was calculated at 36%from the outer diameter of the hollow fiber membrane, the packing numberof the hollow fiber membranes and the inner diameter of the module case.

The above-mentioned module case equipped fluid-tight with bonding jigsat both edges was centrifugally molded using a two-liquid type epoxybonding agent. After the centrifugal molding, the bonding jigs and thepolypropylene rods were removed, and the bonded portion at the sealededge was cut to open the hollow portion of the hollow fibers. Asdescribed above, a module comprising a bundle of wavy hollow fibermembranes was obtained.

The thus-obtained module was treated with ethanol to become hydrophilic,and the pure water flux amount was measured after replacement withwater.

Then, leakage was checked with 100 kPa compressed air. No leak wasobserved.

EXAMPLE 6 (PRODUCTION OF HOLLOW FIBER MEMBRANE MODULE)

1,800 of the hollow fiber membranes obtained in Example 1 were bundled.

Then, the hollow portion of one of the edge faces of the thus-obtainedbundle was sealed, and the bundle was mounted in a cylindrical polyvinylchloride module case having an inner diameter of 83 mmφ and a length of1,000 mm. On the sealed edge of the hollow fiber membrane, only abonding jig was attached fluid-tight to the module case. On the otheredge of the hollow fiber membrane, a total of five polypropylene rods,each having an outer diameter of 11 mmφ, were arranged in parallel tothe hollow fiber membrane and then a bonding jig was attachedfluid-tight to the module case. In this case, the packing ratio wascalculated at 41% from the outer diameter of the hollow fiber membrane,the packing number of the hollow fiber membranes and the inner diameterof the module case.

The above-mentioned module case equipped fluid-tight with bonding jigsat both edges was centrifugally molded using a two-liquid type epoxybonding agent. After the centrifugal molding, the bonding jigs and thepolypropylene rods were removed, and the bonded portion at the sealededge was cut to open the hollow portion of the hollow fibers. Asdescribed above, a module comprising a bundle of wavy hollow fibermembranes was obtained.

The thus-obtained module was treated with ethanol to become hydrophilic,and the pure water flux amount was measured after replacement withwater.

Then, leakage was checked with 100 kPa compressed air. No leak wasobserved.

EXAMPLE 7 (PRODUCTION OF HOLLOW FIBER MEMBRANE MODULE)

Four bundles each comprising 1,440 of the porous hollow fiber membranesobtained in Example 1 were prepared.

Then, after sealing the hollow portion of one of the edge faces of eachbundle, the four bundles were mounted in a cylindrical SUS-304 modulecase having an inner diameter of 150 mmφ and a length of 1,500 mm. Onthe sealed edge of hollow fiber membrane, only a bonding jig wasattached fluid-tight to the module case. On the other edge of the hollowfiber membrane, a total of 37 polypropylene rods, each having an outerdiameter of 10 mmφ, were arranged in parallel to the hollow fibermembrane and then a bonding jig was attached fluid-tight to the modulecase. In this case, the packing ratio was calculated at 40% from theouter diameter of the hollow fiber membrane, the packing number of thehollow fiber membranes and the inner diameter of the module case.

The above-mentioned module case equipped with the bonding jigs on bothedges was centrifugally molded using a silicone bonding agent[TSE-3337(trade name) manufactured and sold by Toshiba Silicone Co.,Ltd.].

After the centrifugal molding, the bonding jigs and the polypropylenerods were removed, and then the bonded portion of the sealed edge wascut to open the hollow portions of the hollow fiber membranes after thesilicone bonded portion was sufficiently cured. As a result, a hollowfiber membrane module comprising a bundle of wavy hollow fiber membraneswas obtained.

Next, the module was mounted in a pressuring container for a module, andleakage was checked with 100 kPa compressed air after being madehydrophilic with a 50% ethanol aqueous solution and being replaced withwater. No leak was observed. At the time, a module weight was alsomeasured in the state that the module was wet.

EXAMPLE 8 (PRODUCTION OF HOLLOW FIBER MEMBRANE MODULE)

A hollow fiber membrane module was prepared in substantially the samemanner as described in Example 5 except that the hollow fiber membraneobtained in Example 2 was used. The packing ratio of the thus-obtainedmodule was 36%.

After the pure water flux amount was measured, leakage was checked with100 kPa compressed air. No leak was observed.

EXAMPLE 9 (PRODUCTION OF HOLLOW FIBER MEMBRANE MODULE)

A hollow fiber membrane module was prepared in substantially the samemanner as described in Example 6 except that the hollow fiber membraneobtained in Example 2 was used. The packing ratio of the thus-obtainedmodule was 41%.

After the pure water flux amount was measured, leakage was checked with100 kPa compressed air. No leak was observed.

EXAMPLE 10 (PRODUCTION OF HOLLOW FIBER MEMBRANE MODULE)

A hollow fiber membrane module was prepared in substantially the samemanner as described in Example 7 except that the hollow fiber membraneobtained in Example 2 was used. The packing ratio of the thus-obtainedmodule was 40%.

After the pure water flux amount was measured, leakage was checked with100 kPa compressed air. No leak was observed. In addition, a moduleweight was also measured in the state the module was wet.

EXAMPLE 11 (PRODUCTION OF HOLLOW FIBER MEMBRANE MODULE)

A hollow fiber membrane module was prepared in substantially the samemanner as described in Example 5 except that the hollow fiber membraneobtained in Example 3 was used. The packing ratio of the thus-obtainedmodule was 36%.

After the pure water flux amount was measured, leakage was checked with100 kPa compressed air. No leak was observed.

EXAMPLE 12 (PRODUCTION OF HOLLOW FIBER MEMBRANE MODULE)

A hollow fiber membrane module was prepared in substantially the samemanner as described in Example 6 except that the hollow fiber membraneobtained in Example 3 was used. The packing ratio of the thus-obtainedmodule was 41%.

After the pure water flux amount was measured, leakage was checked with100 kPa compressed air. No leak was observed.

EXAMPLE 13 (PRODUCTION OF HOLLOW FIBER MEMBRANE MODULE)

A hollow fiber membrane module was prepared in substantially the samemanner as described in Example 5 except that the hollow fiber membraneobtained in Example 4 was used. The packing ratio of the thus-obtainedwas 36%.

After the pure water flux amount was measured, leakage was checked with100 kPa compressed air. No leak was observed.

EXAMPLE 14 (PURIFYING METHOD/THIS INVENTION)

Using the hollow fiber membrane module obtained in Example 5, anoperation was conducted. As raw water, a model liquid (a mixed solutionof bentonite and humic acid [bentonite concentration: 1,000 mg/l, humicacid concentration: 500 mg/l in terms of a total organic carbon amount(TOC)] at the time of production) was used. As shown in FIG. 1, rawwater (1) was fed under pressure into a hollow fiber membrane module (4)through a circulation tank (2) by using a raw water feed pump (3). Theresultant filtrate was stored in a filtrate tank (5). At the time ofback wash, the filtrate in the filtrate tank (5) was fed into the hollowfiber membrane module by using a back wash pump (6).

In addition, air-scrubbing was conducted by supplying compressed airgenerated in a compressor (7) to a raw water inlet of the hollow fibermembrane module.

The filtration was conducted according to a cross-flow type filtrationin which raw water (1) was fed into a hollow fiber membrane module (4)at a constant flow of 1.8 [l/min·module·25° C.] so as to be a ratio of amembrane filtration flow to a water circulation flow of 1/1, andconducted according to an external pressure filtration with a constantfiltration flow, i.e., a filtrate amount of 0.9 [l/min·module·25° C.].

The operation was performed by repeating a 10 minute filtration followedby back wash with filtrate at a flow of 1.5 [l/min·module·25° C.] for 20seconds and conducting air-scrubbing with compressed air at a flow of1.5 [l/min·module·25° C.] for one minute every one hour. The turbidityof raw water was 770 degrees.

The total amount of the filtrate permeating the membrane during thefiltration step was 9 liters. The amount of suspended solidsaccumulating was 5.87.

The turbidity and fine particle mean diameter of the model liquidmeasured just after the filtration operation were 1,000 degrees and 0.9to 30 μm (medium value: 9 μm), respectively. After the above-mentionedfiltration test was continuously conducted for 20 days, the filtrationpressure was 1.2 times the initial value, the average turbidity amongdays was 770 degrees, and the diameter of fine particle was 0.9 to 30 μm(medium value: 9 μm) as well as the initial value.

After the filtration test was completed, the module was taken out fromthe device to check leakage. No leak was observed.

Further, the above-mentioned hollow fiber membrane module was washedwith a sodium hypochlorite aqueous solution, a sodium hydroxide aqueoussolution, an oxalic acid aqueous solution and a nitric acid aqueoussolution until the recoverability was saturated. When the pure waterflux amount was measured, it was 98% of the initial value.

Subsequently, when the hollow fiber membrane module was dismantled toobserve the membrane outer surface of the hollow fiber membrane with ascanning electronic microscope (magnification: 5,000 fold), the damageof the membrane surface was negligible.

EXAMPLE 15 (PURIFYING METHOD/THIS INVENTION)

Using the hollow fiber membrane module obtained in Example 5, anoperation was conducted. As raw water, river surface water having anaverage turbidity among days of 1 and a fine particle diameter of from 5to 200 μm (medium value: 50 μm) was used. The filtration was conductedusing cross-flow type filtration in which raw water was fed into ahollow fiber membrane at a constant flow of 3.0 [l/min·module·25° C.] soas to be a ratio of membrane filtration flow to a water circulation flowof 1/1, according to an external pressure filtration operation with aconstant filtration flow, i.e., a filtrate amount of 1.5[l/min·module·25° C.].

The operation was performed by repeating a 20 minute filtration followedby back wash with filtration water at a flow of 2.5 [l/min·module·25°C.] for 20 seconds and conducting air-scrubbing with compressed air at aflow of 7 [Nl/min·module·25° C.] for one minute every hour. Theturbidity of raw water was 1.0 degree. The total amount of the filtratepermeating the membrane during the filtration step was 30 liters. Theamount of suspended solids accumulating was 0.025.

As the filtration operation proceeded, the filtration pressure graduallyincreased and reached twice the initial filtration pressure in the fifthmonth.

After the operation, the module was taken out from the device to checkleakage. No leak was observed.

Further, the above-mentioned hollow fiber membrane module was washedwith a sodium hypochlorite solution, a sodium hydroxide solution, anoxalic acid solution and a nitric acid solution until the recoverabilitywas saturated. When the pure water flux amount was measured, it was 96%of the initial value.

Subsequently, when the hollow fiber membrane module was dismantled toobserve the membrane outer surface of the hollow fiber membrane with ascanning electronic microscope (magnification: 5,000 fold), the damageof the membrane surface was negligible.

EXAMPLE 16 (PURIFYING METHOD/THIS INVENTION)

Using the hollow fiber membrane module obtained in Example 6, apurifying operation was conducted. As raw water, river surface waterhaving a turbidity of 0.1 to 5 degrees (average: 2.4 degrees), a fineparticle diameter in water of from 0.9 to 30 μm (medium value: 9 μm) anda temperature of 12° C. was used.

The filtration was conducted using cross-flow type filtration in whichraw water was fed into a hollow fiber membrane at a constant flow of 2.6[m³/hr·module·25° C.] so as to a ratio of a membrane filtration flow toa water circulation flow of 1/1, with a constant filtration flow, i.e.,a filtrate amount of 1.3 [m³/hr·module·25° C.].

The operation was performed by repeating a 20 minute filtration followedby back wash with filtrate for 20 seconds and conducting back wash withfiltrate at a flow of 1.3 [m³/hr·module·25° C.] and air-scrubbing withcompressed air at a flow of 2 [Nm³/hr·module·25° C.] simultaneously for2 minutes every hour. The turbidity of raw water was 2.4 degrees. Thetotal amount of the filtrate permeating the membrane during thefiltration step was 0.43 m³. The amount of suspended solids accumulatingwas 0.15.

After 12 month operation under the above-mentioned conditions, thetrans-membrane pressure became 1.3 times the initial value. After theoperation, the module was taken out from the device to check leakage. Noleak was observed. Subsequently, the hollow fiber membrane module afterthe operation was dismantled and a single hollow fiber was subjected tochemical wash with a mixed solution of a sodium hypochlorite solutionand a sodium hydroxide solution and a mixed solution of an oxalic acidsolution and a nitric acid solution. When a pure water flux amount wasmeasured, it corresponded to 95% of that of unused membrane. When themembrane outer surface of the hollow fiber membrane was observed with ascanning electronic microscope (magnification: 5,000 fold), the damageof the membrane outer surface was negligible.

EXAMPLE 17 (PURIFYING METHOD/THIS INVENTION)

Using the hollow fiber membrane module obtained in Example 6, anoperation was conducted in substantially the same manner as Example 16except that the operation was performed by conducting a membranefiltration for 60 minutes and conducting back wash with filtrate andair-scrubbing with compressed air simultaneously for 2 minutes. Theturbidity of raw water was 2.4 degrees. The total amount of filtratepermeating the membrane during the filtration step was 1.3 m³. Theamount of suspended solids accumulating was 0.44.

After 6 month operation under the above-mentioned conditions, thetrans-membrane pressure became 1.4 times the initial value. After theoperation, the module was taken out from the device to check leakage. Noleak was observed. Subsequently, the hollow fiber membrane module afterthe operation was dismantled, and a single hollow fiber was subjected tochemical wash with a mixed solution of a sodium hypochlorite solutionand a sodium hydroxide solution and a mixed solution of an oxalic acidsolution and a nitric acid solution. When a pure water flux amount wasmeasured, it corresponded to 95% of that of unused membrane. When themembrane outer surface of the hollow fiber membrane was observed with ascanning electronic microscope (magnification: 5,000 fold), the damageof the membrane surface was negligible.

EXAMPLE 18 (PURIFYING METHOD/THIS INVENTION)

Using the membrane module obtained in Example 7, an operation wasconducted. As raw water, river surface water having turbidity of from 1to 3 degrees (average: 1.8 degrees) and a fine particle diameter of from2 to 50 μm (medium value: 22 μm) was used. As shown in FIG. 2, raw water(11) was fed under pressure into a hollow fiber membrane module (14)through a circulation tank (12) by using a raw water feed pump (13). Theresultant filtrate was stored in a filtrate tank (15). The raw water fedinto the hollow fiber membrane module (14) under pressure was mixed withan ozone gas generated by an ozone generator (18) to obtain aconcentration of ozone water of 0.3 mg/l at the filtrate side. At thetime of back wash, the filtrate in the filtrate tank (15) was fed intothe hollow fiber membrane module (14) by using a back wash pump (16).

The filtration was conducted according to a dead-end type one underconstant pressure, i.e., a trans-membrane pressure of 30 kPa, in whichthe raw water (11) was supplied to the hollow fiber membrane module (14)and concentrated water was not discharged except for discharge ofozone-containing air. At the time of back wash, a back wash pressure was50 kPa.

After the above filtration test was continuously conducted for 10 days,the filtrate amount recovered was 70% of the initial value.

Subsequently, when the hollow fiber membrane module was taken out fromthe filtration operation device and weighed, its weight had increased to115% of the initial weight.

Again, this module was installed in the operation device shown in FIG. 2to conduct air-scrubbing (supplying water amount: 3 m³/Hr, supplying airamount: 5 Nm³/Hr, air-scrubbing period: 5 min) with a valve on thefiltrate side of the hollow fiber membrane module shut while feeding rawwater.

After the air-scrubbing, the hollow fiber membrane module was againweighed. The weight was 103% of the initial weight.

This means that suspended solids were discharged by air-scrubbing in anamount corresponding to 12% of the weight of the hollow fiber membranemodule.

Further, the above hollow fiber membrane module was subjected to leakagecheck. No leak was observed.

The above hollow fiber membrane module was dismantled to observe thestate of bonded and fixed portions. It was confirmed that both theexternal and central portions of the hollow fiber membrane bundle weresufficiently filled up with the bonding agent.

EXAMPLE 19 (PURIFYING METHOD/THIS INVENTION)

Using the hollow fiber membrane module obtained in Example 5, theoperation was conducted. As raw water, river surface water havingturbidity of from 3 to 340 degrees (average: 120 degrees) and a fineparticle diameter of from 2 to 130 μm (medium value: 43 μm) was used.

The filtration was conducted according to a cross flow filtration typeoperation, in which raw water was fed into a hollow fiber membranemodule at a constant flow of 8.0 [l/min·module·25° C.] so as to be aratio of an amount of water filtered through the membrane to that ofwater circulating of 1/1, with a constant filtration flow, i.e., afiltrate amount of 4.0 [l/min·module·25° C.].

The operation was performed by repeating a 10 minute filtration followedby conducting back wash with filtrate at a flow of 6.0 [l/min·module·25°C.] and air-scrubbing with compressed air at a flow of 8[Nm³/hr·module·25° C.] simultaneously for one minute. The turbidity ofraw water was 120 degrees. The total filtrate permeating membrane duringthe filtration step was 40 liters. The amount of suspended solidsaccumulating was 4.1.

After the operation was performed for 2 months under the above-mentionedconditions, raw water having high turbidity of 340 degrees was fed for 2days. Therefore, just for these two days and the following day, i.e.,for three days in total, the operation conditions were changed to themanner wherein back wash and air-scrubbing were simultaneously conductedafter 5 minute filtration. At this time, the amount of suspended solidsaccumulating was 5.8 since physical wash was conducted every 5 minutes;while, the amount of suspended solids accumulating was 11.6 in theoperation of 10 minute filtration followed by one minute physical wash.

After operating for a total of 3 months, the trans-membrane pressurereached 1.3 times the initial value. Then, the module was taken out fromthe device to check leakage. No leak was observed. Subsequently, thehollow fiber membrane module after the operation was dismantled and asingle hollow fiber was subjected to chemical wash with a mixed solutionof a sodium hypochlorite solution and a sodium hydroxide solution and amixed solution of an oxalic acid solution and a nitric acid solution.When a pure water flux amount was measured, it corresponded to 95% ofthat of unused membrane. When the membrane outer surface of the hollowfiber membrane was observed with a scanning electronic microscope(magnification: 5,000 fold), the damage of the membrane outer surfacewas negligible.

EXAMPLE 20 (PURIFYING METHOD/THIS INVENTION)

Using the hollow fiber membrane module obtained in Example 5, afiltration operation was conducted. As raw water, river surface waterhaving turbidity of from 0.1 to 3 degrees (average: 1.2 degrees), a fineparticle diameter of from 0.5 to 30 μm (medium value: 7 μm) and atemperature of 18° C. was used.

The filtration was conducted using a cross flow filtration typeoperation, in which raw water was fed into a hollow fiber membranemodule at a constant flow of 3.0 [m³/hr·module·25° C.] so as to be aratio of an amount of water filtered through the membrane to that ofwater circulating of 1/1, with a constant filtration flow, i.e., afiltrate amount of 1.5 [m³/hr·module·25° C.].

The operation was performed by repeating a filtration cycle in whichback wash with filtrate at a flow of 1.5 [m³/hr·module·25° C.] andair-scrubbing with compressed air at a flow of 2 [Nm³/hr·module·25° C.]were simultaneously conducted for two minutes after 30 minutefiltration. The turbidity of raw water was 1.2 degrees. The totalfiltrate permeating membrane during the filtration step was 0.75 m³. Theamount of suspended solids accumulating was 0.13.

After the operation was performed for 10 months under theabove-mentioned operation conditions, the trans-membrane pressure was1.2 times the initial value. Then, the module was taken out from theapparatus to check leakage. No leak was observed. Subsequently, thehollow fiber membrane module after the operation was dismantled and asingle hollow fiber was subjected to chemical wash with a mixed solutionof a sodium hypochlorite solution and a sodium hydroxide solution and amixed solution of an oxalic acid solution and a nitric acid solution.When a pure water flux amount was measured, it corresponded to 96% ofthat of unused membrane. When the membrane outer surface of the hollowfiber membrane was observed with a scanning electronic microscope(magnification: 5,000 fold), the damage of the membrane outer surfacewas negligible.

EXAMPLE 21 (PURIFYING METHOD/THIS INVENTION)

Using the hollow fiber membrane module obtained in Example 5, afiltration operation was conducted. As raw water, river surface waterhaving turbidity of from 0.1 to 3 degrees (average: 1.2 degrees), a fineparticle diameter of from 0.5 to 30 μm (medium value: 7 μm) and atemperature of 18° C. was used.

The filtration was conducted using a cross flow filtration typeoperation, in which raw water was fed into a hollow fiber membranemodule at a constant flow of 3.0 [m³/hr·module·25° C.] to obtain a ratioof an amount of water filtered through the membrane to that of watercirculating of 1/1, with a constant filtration flow, i.e., a filtrateamount of 1.5 [m³/hr·module·25° C.].

The operation was performed by repeating a filtration cycle in whichafter 30 minute filtration, back wash with filtrate at a flow of1.5[m³/hr·module·25° C.] and air-scrubbing with compressed air at a flowof 2 [Nm³/hr·module·25° C.] were simultaneously conducted for 1 minuteand a flushing with raw water was conducted at a flow of 2.5[m³/hr·module·25° C.] for one minute. The turbidity of raw water was 1.2degrees. The total filtrate permeating membrane during the filtrationstep was 0.75 m³. The amount of suspended solids accumulating was 0.13.

After the operation was performed for 5 months under the above-mentionedoperation conditions, the trans-membrane pressure was 1.2 times theinitial value. Then, the module was taken out from the device to checkleakage. No leak was observed. Subsequently, the hollow fiber membranemodule after the operation was dismantled and a single hollow fiber wassubjected to chemical wash with a mixed solution of a sodiumhypochlorite solution and a sodium hydroxide solution and a mixedsolution of an oxalic acid solution and a nitric acid solution. When apure water flux amount was measured, it corresponded to 95% of that ofunused membrane. When the membrane outer surface of the hollow fibermembrane was observed with a scanning electronic microscope(magnification: 5,000 fold), the damage of the membrane surface wasnegligible.

EXAMPLE 22 (PURIFYING METHOD/COMPARISON)

An operation was performed in parallel with Example 14 undersubstantially the same conditions as described in Example 14 except thatthe hollow fiber membrane module obtained in Example 8 was used. Afterthe operation was conducted for 20 days, the trans-membrane pressure was3.5 times the initial value.

After the filtration test, the leakage was checked. No leak wasobserved.

Further, after the above-mentioned membrane module was washed with asodium hydroxide aqueous solution, a sodium hypochlorite aqueoussolution, an oxalic acid aqueous solution and a nitric acid aqueoussolution until the recoverability was saturated, the flux amount of purewater was measured. It was 66% of the initial value.

Subsequently, the hollow fiber membrane module was dismantled and themembrane outer surface of the hollow fiber membrane was observed with ascanning electronic microscope (magnification: 5,000 fold). It wasobserved that about 75% of the membrane surface was rough and a part ofthe open pores on the membrane surface was covered. This was supposed tobe a factor in causing a decrease of the water flux amount.

EXAMPLE 23 (PURIFYING METHOD/COMPARISON)

An operation was performed in parallel with Example 15 undersubstantially the same conditions as described in Example 15 except thatthe hollow fiber membrane module obtained in Example 8 was used.

The filtration pressure gradually increased as the filtration operationproceeded. The trans-membrane pressure reached 3 times the initial valuein the second month and the fourth month of the filtration operation.Therefore, the module was subjected to chemical wash with a sodiumhydroxide aqueous solution, a sodium hypochlorite aqueous solution, anoxalic acid aqueous solution and a nitric acid aqueous solution.

When the total operation term was 5 months, the hollow fiber membranemodule was taken out from the device to check the leakage. No leak wasobserved.

Further, after the above-mentioned membrane module was washed with asodium hydroxide aqueous solution, a sodium hypochlorite aqueoussolution, an oxalic acid aqueous solution and a nitric acid aqueoussolution until the recoverability was saturated, the flux amount of purewater was measured. It was 72% of the initial value.

Subsequently, the hollow fiber membrane module was dismantled and themembrane outer surface of the hollow fiber membrane was observed with ascanning electronic microscope (magnification: 5,000 fold). It wasobserved that about 70% of the membrane surface was rough and a part ofthe open pores on the membrane surface was covered. This was supposed tobe a factor in causing a decrease of the water flux amount.

EXAMPLE 24 (PURIFYING METHOD/COMPARISON)

An operation was performed in parallel with Example 16 undersubstantially the same conditions as described in Example 16 except thatthe hollow fiber membrane module obtained in Example 9 was used.

After operating for 6 months, the trans-membrane pressure became 2.0times the initial value. Judging that it would be impossible to continuethe filtration operation further, the hollow fiber membrane module wasdismantled. A single fiber of the hollow fiber membrane module wasdismantled and subjected to chemical wash with a mixed solution of asodium hypochlorite aqueous solution and a sodium hydroxide aqueoussolution, and with a mixed solution of an oxalic acid aqueous solutionand a nitric acid aqueous solution, and the flux amount of pure water ofthe single fiber was measured. It corresponded to 80% of that of anunused membrane. The outer surface of the membrane was observed with ascanning electronic microscope (magnification: 5,000 fold). It wasobserved that approximately 70% of the membrane surface was rough and apart of open pores on the membrane surface was covered. This wassupposed to be a factor in causing a decrease of the water flux amount.

EXAMPLE 25 (PURIFYING METHOD/COMPARISON)

An operation was performed in parallel with Example 17 undersubstantially the same conditions as described in Example 17 except thatthe hollow fiber membrane module obtained in Example 9 was used.

After operating for 4 months, the trans-membrane pressure reached 2.0times the initial value. Judging that it would be impossible to conductthe filtration operation further, the hollow fiber membrane module wasdismantled. After a single fiber of the hollow fiber membrane moduledismantled was subjected to chemical wash with a mixed solution of asodium hypochlorite aqueous solution and a sodium hydroxide aqueoussolution, and with a mixed solution of an oxalic acid aqueous solutionand a nitric acid aqueous solution, the flux amount of pure water of thesingle fiber was measured. It corresponded to 82% of that of an unusedmembrane. The outer surface of membrane was observed with a scanningelectronic microscope (magnification: 5,000 fold). It was observed thatapproximately 70% of the membrane surface was rough and a part of openpores on the membrane surface was covered. This was supposed to be afactor in causing a decrease of the water flux amount.

EXAMPLE 26 (PURIFYING METHOD/COMPARISON)

A filtration operation was conducted under substantially the sameconditions as described in Example 18 except that the hollow fibermembrane module obtained in Example 10 was used.

After the above filtration test was continuously conducted for 10 days,the filtrate amount recovered was 60% of the initial value.

Subsequently, when the hollow fiber membrane module was taken out fromthe filtration operation apparatus and weighed, its weight was increasedto 120% of the initial weight.

Again, this module was installed in the filtration operation device toconduct air-scrubbing (supplying water amount: 3 m³/Hr, supplying airamount: 5 Nm³/Hr, air-scrubbing period: 5 min) with a valve on thefiltrate side of the hollow fiber membrane module shut while feeding rawwater.

After the air-scrubbing, the hollow fiber membrane module was againweighed. The weight was 115% of the initial weight.

This means that suspended solids were discharged by air-scrubbing in anamount corresponding to 5% of the weight of the hollow fiber membranemodule.

Further, the above hollow fiber membrane module was subjected to leakagecheck. No leak was observed.

The above hollow fiber membrane module was dismantled to observe thestate of bonded and fixed portions. It was confirmed that a part of thecentral portion of the hollow fiber membrane bundle was not sufficientlyfilled up with the bonding agent.

EXAMPLE 27 (PURIFYING METHOD/COMPARISON)

The filtration operation was performed in parallel with Example 19 undersubstantially the same conditions as described in Example 19 except thatthe hollow fiber membrane module obtained in Example 8 was used.

When the filtration operation was performed for 2 months under theabove-mentioned conditions, raw water having high turbidity of 340degrees was fed for 2 days. Therefore, just for these two days and thefollowing day, i.e., for three days in total, the operation conditionswere changed to the manner wherein back wash and air-scrubbing weresimultaneously conducted after 5 minute filtration. At this time, theamount of suspended solids accumulating was 5.8 since physical wash wasconducted every 5 minutes; while, the amount of suspended solidsaccumulating was 11.6 in the operation of 10 minute filtration followedby one minute physical wash.

After operating for a total of 3 months, the trans-membrane pressurereached 2.5 times the initial value. Then, the module was taken out fromthe device to check leakage. No leak was observed. Subsequently, thehollow fiber membrane module after the operation was dismantled and asingle hollow fiber was subjected to chemical wash with a mixed solutionof a sodium hypochlorite solution and a sodium hydroxide solution, and amixed solution of an oxalic acid solution and a nitric acid solution.When a pure water flux amount was measured, it corresponded to 74% ofthat of an unused membrane.

When the membrane outer surface of the hollow fiber membrane wasobserved with a scanning electronic microscope (magnification: 5,000fold), approximately 70% of the membrane surface was rough and a part ofopen pores on the membrane surface was covered. This was supposed to bea factor in causing a decrease of the water flux amount.

EXAMPLE 28 (PURIFYING METHOD/THIS INVENTION)

An operation was performed under substantially the same conditions asdescribed in Examples 14 and 22 except that the hollow fiber membranemodule obtained in Example 11 was used. After operating for 20 days, thetrans-membrane pressure reached 2.9 times the initial value. After thefiltration test, leakage was checked. No leak was observed.

Further, the above-mentioned membrane module was washed with a sodiumhydroxide aqueous solution, a sodium hypochlorite aqueous solution, anoxalic acid aqueous solution and a nitric acid aqueous solution untilthe recoverability was saturated. When the flux amount of pure water wasmeasured, it was 87% of the initial value.

Then, the outer surface of membrane was observed with a scanningelectronic microscope (magnification: 5,000 fold). It was observed thatapproximately 20% of the membrane surface was rough and a part of openpores on the membrane surface was slightly covered. This was supposed tobe a factor in causing a decrease of the water flux amount.

EXAMPLE 29 (PURIFYING METHOD/THIS INVENTION)

An operation was performed in parallel with Examples 15 and 23 undersubstantially the same conditions as described in Examples and 23 exceptthat the hollow fiber membrane module obtained in Example 11 was used.

The filtration pressure gradually increased as the filtration operationproceeded. The trans-membrane pressure reached 3 times the initial valuein the third month of the filtration operation. Therefore, the modulewas subjected to chemical wash with a sodium hydroxide aqueous solution,a sodium hypochlorite aqueous solution, an oxalic acid aqueous solutionand a nitric acid aqueous solution.

When the total operation term was 5 months, the hollow fiber membranemodule was taken out from the device to check the leakage. No leak wasobserved.

Further, the above-mentioned membrane module was washed with sodiumhydroxide aqueous solution, sodium hypochlorite aqueous solution, oxalicacid aqueous solution and nitric acid aqueous solution until therecoverability was saturated. When the flux amount of pure water wasmeasured, it was 79% of the initial value.

Subsequently, the hollow fiber membrane module was dismantled and themembrane outer surface of the hollow fiber membrane was observed with ascanning electronic microscope (magnification: 5,000 fold). It wasobserved that about 20% of the membrane surface was rough and a part ofthe open pores on the membrane surface was covered. This was supposed tobe a factor in causing a decrease of the water flux amount.

EXAMPLE 30 (PURIFYING METHOD/THIS INVENTION)

An operation was performed under substantially the same conditions asdescribed in Examples 14 and 22 except that the hollow fiber membranemodule obtained in Example 12 was used and river surface water having aturbidity of 0.1 to 5 degrees (average: 2.2 degrees), a fine particlediameter of from 0.9 to 30 μm (medium value: 9 μm) and a temperature of12° C. was employed as raw water. The turbidity of raw water was 2.2degrees. The total amount of filtrate permeating the membrane during thefiltration step was 0.43 m³. The amount of suspended solids accumulatingwas 0.13.

After operating for 8 months, the trans-membrane pressure reached 2.0times the initial value. Judging that it would be impossible to conductthe filtration operation further, the hollow fiber membrane module wasdismantled. A single fiber of the dismantled hollow fiber membranemodule was subjected to chemical wash with a mixed solution of a sodiumhypochlorite aqueous solution and a sodium hydroxide aqueous solution,and with a mixed solution of an oxalic acid aqueous solution and anitric acid aqueous solution. When the flux amount of pure water of thesingle fiber was measured, it corresponded to 83% of that of an unusedmembrane. The outer surface of membrane was observed with a scanningelectronic microscope (magnification: 5,000 fold). It was observed thatapproximately 20% of the membrane surface was rough and a part of openpores on the membrane surface was slightly covered. This was supposed tobe a factor in causing a decrease of the water flux amount.

EXAMPLE 31 (PURIFYING METHOD/THIS INVENTION)

An operation was conducted in parallel with Examples 19 and 27 undersubstantially the same conditions as described in Examples 19 and 27except that the hollow fiber membrane module obtained in Example 11 wasused.

When the filtration operation was performed for 2 months under theabove-mentioned conditions, raw water having high turbidity of 340degrees was fed for 2 days. Therefore, just for these two days and thefollowing day, i.e., for three days in total, the operation conditionswere changed to the manner wherein back wash and air-scrubbing weresimultaneously conducted after 5 minute filtration. At this time, theamount of suspended solids accumulating was 5.8 since physical wash wasconducted every 5 minutes; while, the amount of suspended solidsaccumulating was 11.6 in the operation of 10 minute filtration followedby one minute physical wash.

After operating for a total of 3 months, the trans-membrane pressurereached 2.0 times the initial value. Then, the module was taken out fromthe apparatus to check leakage. No leak was observed. Subsequently, thehollow fiber membrane module was dismantled and a single hollow fiberwas subjected to chemical wash with a mixed solution of a sodiumhypochlorite solution and a sodium hydroxide solution and a mixedsolution of an oxalic acid solution and a nitric acid solution. When apure water flux amount was measured, it corresponded to 78% of that ofan unused membrane.

When the membrane outer surface of the hollow fiber membrane wasobserved with a scanning electronic microscope (magnification: 5,000fold), approximately 20% of the membrane surface was rough and a part ofopen pores on the membrane surface was slightly covered. This wassupposed to be a factor in causing a decrease of the water flux amount.

EXAMPLE 32 (PURIFYING METHOD/THIS INVENTION)

A filtration operation was performed at the same time of Example 28under substantially the same conditions as described in Example 28except that the hollow fiber membrane module obtained in Example 13 wasused. After operating for 20 days, the trans-membrane pressure reached3.3 times the initial value. After the filtration operation, leakage waschecked. No leak was observed.

Further, the above-mentioned membrane module was washed with a sodiumhydroxide aqueous solution, a sodium hypochlorite aqueous solution, anoxalic acid aqueous solution and a nitric acid aqueous solution untilthe recoverability was saturated. When the flux amount of pure water wasmeasured, it was 70% of the initial value.

Then, the outer surface of membrane was observed with a scanningelectronic microscope (magnification: 5,000 fold). It was observed thatapproximately 50% of the membrane surface was rough and a part of openpores on the membrane surface was slightly covered. This was supposed tobe a factor in causing a decrease of water flux amount.

The results of Examples 1 to 32 are shown in Tables 2 to 9. As seen formTables 2 to 9, it is apparent that in the purifying methods employing ahollow fiber membrane having an outer diameter of from 0.5 to 3.1 mm,one employing a wavy hollow fiber membrane, which is covered by thescope of the present invention, provides a more stable filtration thanone employing a non-wavy hollow fiber membrane without waves. Further,it is seen that more preferable results can be obtained when thebulkiness is within the preferred range (1.45 to 2.00) or when waveshaving different wavelength and wave height exist together.

TABLE 1 Example 1 Example 2 Example 3 Example 4 Major Minor Major MinorMajor Minor Major Minor Axis Axis Axis Axis Axis Axis Axis Axis nNumber[μm] [μm] Flatness [μm] [μm] Flatness [μm] [μm] Flatness [μm] [μm]Flatness 1 699 677 0.969 713 685 0.961 777 633 0.815 715 679 0.950 2 714693 0.971 715 698 0.976 792 607 0.766 709 681 0.961 3 707 686 0.970 703701 0.997 768 641 0.835 707 676 0.956 4 727 688 0.946 720 682 0.947 799585 0.732 728 682 0.937 5 709 698 0.984 706 680 0.963 755 649 0.860 729683 0.937 Average 711.2 688.4 0.968 711.4 689.2 0.969 778.2 623.0 0.801717.6 680.2 0.948

TABLE 2 Membrane Inner Diameter Thickness Outer Diameter (mm) (mm) (mm)Flatness Fiber Shape Bulkiness Example 1 0.7 0.28 1.25 0.968 UnuniformWave 1.66 Example 2 0.7 0.28 1.25 0.969 Straight 1.43 Example 3 0.7 0.281.25 0.801 Uniform Wave 1.51 Example 4 0.7 0.28 1.25 0.948 Uniform Wave1.44

TABLE 3 Membrane Packing Number Packing Ratio Example 5 Example 1  30036% Example 6 Example 1 1800 41% Example 7 Example 1 5760 40% Example 8Example 2  300 36% Example 9 Example 2 1800 41% Example 10 Example 25760 40% Example 11 Example 3  300 36% Example 12 Example 3 1800 41%Example 13 Example 4  300 36%

TABLE 4 Average Diameter Diameter Accumulating Ascent of Fine of FineAverage Amount of Type of Ratio in Wash Type of Particle ParticleTurbidity Suspended Physical Filtration Recover- Module Filtration [μm][μm] [degree] Solid Wash Test Time Pressure ability Ex. 14 Ex. 5Constant 0.9-30 9 770 5.8 RF/AS 20 days 1.2 fold 98 [%] Flow Ex. 22 Ex.8 Constant 0.9-30 9 770 5.8 RF/AS 20 days 3.5 fold 66 [%] Flow Ex. 28Ex. 11 Constant 0.9-30 9 770 5.8 RF/AS 20 days 2.9 fold 87 [%] Flow Ex.32 Ex. 13 Constant 0.9-30 9 770 5.8 RF/AS 20 days 3.3 fold 70 [%] Flow*RF/AS: Back wash and air-scrubbing were individually performed.

TABLE 5 Average Diameter Diameter Accumulating Ascent of Fine of FineAverage Amount of Type of Ratio in Wash Type of Particle ParticleTurbidity Suspended Physical Filtration Recover- Module Filtration [μm][μm] [degree] Solid Wash Test Time Pressure ability Ex. 15 Ex. 5Constant 5-200 50 1 0.025 RF/AS 5 months 2 fold 96 [%] Flow Ex. 23 Ex. 8Constant 5-200 50 1 0.025 RF/AS 5 months 3 fold 72 [%] Flow Ex. 29 Ex.11 Constant 5-200 50 1 0.025 RF/AS 5 months 3 fold 79 [%] Flow *RF/AS:Back wash and air-scrubbing were individually performed. *Ascent ratioin Filtration Pressure: Ex. 23: Value in 2^(nd) and 4^(th) monthoperation Ex. 29: Value in 3^(rd) month operation

TABLE 6 Average Diameter Diameter Accumulating Ascent of Fine of FineAverage Amount of Type of Ratio in Wash Type of Particle ParticleTurbidity Suspended Physical Filtration Recover- Module Filtration [μm][μm] [degree] Solid Wash Test Time Pressure ability Ex. 16 Ex. 6Constant 0.9-30 9 2.4 0.15 AS-RF 12 months 1.3 fold 95 [%] Flow Ex. 24Ex. 9 Constant 0.9-30 9 2.4 0.15 AS-RF  6 months   2 fold 80 [%] FlowEx. 30 Ex. 12 Constant 0.9-30 9 2.2 0.15 AS-RF  8 months   2 fold 81 [%]Flow *AS-RF: Air-scrubbing and back wash were simultaneously performed.

TABLE 7 Average Diameter Diameter Accumulating Ascent of Fine of FineAverage Amount of Type of Ratio in Wash Type of Particle ParticleTurbidity Suspended Physical Filtration Recover- Module Filtration [μm][μm] [degree] Solid Wash Test Time Pressure ability Ex. 17 Ex. 6Constant 0.9-30 9 2.4 0.44 AS-RF 6 months 1.4 fold 95 [%] Flow Ex. 25Ex. 9 Constant 0.9-30 9 2.4 0.44 AS-RF 3 months   2 fold 82 [%] Flow*AS-RF: Air-scrubbing and back wash were simultaneously performed.

TABLE 8 Average Diameter Diameter of Fine of Fine Average Changing Typeof Descent Ratio Type of Particle Particle Turbidity Ratio in Physicalin Permeation Module Filtration [μm] [μm] [degree] Module Weight WashTest Time Flux Ex. 18 Ex. 7 Constant 0.9-30 9 1.0-3 115% → 103% RF 10days 70 [%] Pressure Ex. 26 Ex. 10 Constant 0.9-30 9 1.0-3 120% → 115%RF 10 days 60 [%] Pressure *RF: Only back wash was performed. *ChangingRatio in Module Weight: Weight % of weight just after test to initialweight → Weight % of weight after air-scrubbing to initial weight

TABLE 9 Average Diameter Diameter Accumulating Ascent of Fine of FineAverage Amount of Type of Ratio in Wash Type of Particle ParticleTurbidity Suspended Physical Filtration Recover- Module Filtration [μm][μm] [degree] Solid Wash Test Time Pressure ability Ex. 19 Ex. 5Constant 2-130 43 120 (340) 4.1 (5.7) AS-RF 3 months 1.2 fold 95 [%]Flow Ex. 27 Ex. 8 Constant 2-130 43 120 (340) 4.1 (5.7) AS-RF 3 months2.5 fold 74 [%] Flow Ex. 31 Ex. 11 Constant 2-130 43 120 (340) 4.1 (5.7)AS-RF 3 months   2 fold 78 [%] Flow *AS-RF: Air-scrubbing and back washwere simultaneously performed.

What is claimed is:
 1. A method for purifying an aqueous suspensioncomprising: feeding the aqueous suspension containing fine particlescomprising an inorganic component from outer surfaces to inner surfacesof wavy hollow fiber membranes, which are collected with a bulkiness offrom 1.45 to 2.00, and which each have an outer diameter from 0.5 to 3.1mm and a flatness of from 0.8 to 1.0 to filter said aqueous suspension;the hollow fiber membranes having a bulkiness and being mounted orpacked so as to substantially prevent rubbing of the membranes againsteach other during physical washing, and physically washing the hollowfiber membranes.
 2. The method according to claim 1, wherein filtrationtime is controlled so an amount of suspended solids accumulating is from0.005 to 10, wherein the amount of suspended solids accumulating isdefined by the following formula: amount of suspended solidsaccumulating=(raw water turbidity [degree])×(total amount of filtratepermeating membrane in filtration time {m³})/(membrane surface area{m²}), the raw water turbidity [degree] meaning an average turbidity ofraw water, which can be obtained by measuring turbidity for plural daysaccording to JIS K 0101 9.2 and averaging the obtained value.
 3. Themethod according to claim 1, wherein the physically washing is at leastone selected from the group consisting of back wash, air-scrubbing andflushing.
 4. The method according to claim 1, wherein the physicallywashing comprises back wash followed by air-scrubbing.
 5. The methodaccording to claim 1, wherein the physically washing comprisesair-scrubbing and back wash performed simultaneously after thefiltration.
 6. The method according to claim 1, wherein the physicallywashing is comprises: air-scrubbing and back wash performedsimultaneously after the filtration; and flushing.
 7. The methodaccording to claim 1, wherein the wavy hollow fiber membranes each havea flatness of from 0.9 to 1.0.
 8. The method according to claim 1,wherein the wavy hollow fiber membranes each have a flatness of from0.95 to 1.0.
 9. The method according to claim 1, wherein the wavy hollowfiber membranes each have an outer diameter of from 0.7 to 2.5 mm. 10.The method according to claim 1, wherein the wavy hollow fiber membraneseach have an outer diameter of from 1.0 to 2.5 mm.
 11. The methodaccording to claim 1, wherein the hollow fiber membranes each have aninner diameter of from 0.3 to 1.7 mm.
 12. The method according to claim1, wherein the wavy hollow fiber membrane each have an inner diameter of0.3 to 1.7 mm and membrane thickness of from 0.1 to 0.7 mm.
 13. Themethod according to claim 1, wherein the wavy hollow fiber membraneseach have an inner diameter of from 0.3 to 1.7 mm and membrane thicknessof from 0.1 to 0.7 mm, the membrane being oriented in the same directionand mounted with a packing ratio of from 35 to 55%.
 14. A method forpurifying an aqueous suspension comprising: feeding the aqueoussuspension containing fine particles comprising an inorganic componentfrom outer surfaces to inner surfaces of wavy hollow fiber membranes,each having an outer diameter from 0.5 to 3.1 mm, to filter said aqueoussuspension wherein filtration time is controlled so an amount ofsuspended solids accumulating is from 0.005 to 10, the hollow fibermembranes having a bulkiness and being mounted or packed so as tosubstantially prevent rubbing of the membranes against each other duringphysical washing, wherein the amount of suspended solids accumulating isdefined by the following formula: amount of suspended solidsaccumulating=(raw water turbidity {degree})×(total amount of filtratepermeating membrane in filtration time {m³})/(membrane surface area{m²}), the raw water turbidity {degree} meaning an average turbidity ofraw water, which can be obtained by measuring turbidity for plural daysaccording to JIS K 0101 9.2 and averaging the obtained value; andphysically washing the hollow fiber membranes.
 15. A method forpurifying an aqueous suspension comprising: feeding the aqueoussuspension containing fine particles comprising an inorganic componentfrom outer surfaces to inner surfaces of wavy hollow fiber membranes,which are collected with a bulkiness of from 1.45 to 2.00 and mounted ina module case with a packing ratio of from 35 to 55%, and which eachhave an outer diameter from 0.5 to 3.1 mm and a flatness of from 0.8 to1.0 to filter said aqueous suspension; the hollow fiber membranes havinga bulkiness and being mounted or packed so as to substantially preventrubbing of the membranes against each other during physical washing, andphysically washing the hollow fiber membranes.